Phosphate in a Reef Tank: Target and Control

What the sources say

Phosphate targets are tiny numbers, and small differences matter.

Bulk Reef Supply notes that some mature tanks with large corals do well at 0.1 mg/L or more.

Why it matters

Holmes-Farley explains two effects. Phosphate can slow the rate at which corals and coralline algae build their skeletons, and it’s often the nutrient that limits how fast nuisance algae can grow. Bulk Reef Supply cites research suggesting stony coral skeletons get less dense above about 0.05 mg/L.

Too little is a problem too. At zero, corals can pale and stall, and some reef keepers report other nuisances, such as cyanobacteria, moving in.

Phosphate or phosphorus? Check your kit

Some kits and meters report phosphorus (P) rather than phosphate (PO₄). They measure the same thing in different units. Hanna gives the conversion: multiply phosphorus by 3.066 to get phosphate. So 30 ppb phosphorus is about 92 ppb, or 0.09 mg/L, phosphate.

Hanna’s ultra-low-range phosphorus checker, for example, reports phosphorus in ppb. If you compare that number directly with a phosphate target, you’ll think you’re three times lower than you are.

Bringing high phosphate down

Phosphate comes from food, so the first fix is the same as for nitrate: feed carefully and keep filtration clean. After that, there are more ways to remove phosphate directly than nitrate.

• Feeding and filtration: feed less, change filter socks often, keep the skimmer working.
• RODI water: tap water can contain phosphate. Use RODI water for top-up and water changes.
• GFO (granular ferric oxide): binds phosphate slowly. It works best in a media reactor.
• Lanthanum chloride: a liquid that binds phosphate quickly. It clouds the water for a while, and Bulk Reef Supply advises changing socks and emptying the skimmer during treatment.
• Refugium or algae scrubber: takes phosphate and nitrate up into algae that you harvest.

Slow is safe

Bulk Reef Supply warns that a sudden drop in phosphate can seriously stress corals, especially in tanks used to higher levels. Use less media than the label maximum to start with, test every few days, and back off as you approach your target.

Watch for phosphate falling to zero while nitrate stays high. That imbalance can bring its own problems. If both are dropping too fast, ease off your removal before it bottoms out.

Is 0.1 phosphate too high?

For a mixed reef, Red Sea suggests about 0.1 mg/L, so not necessarily. For an SPS-dominated tank, it’s on the high side. Watch your corals and algae.

Can I use GFO and lanthanum together?

Public aquariums reportedly use lanthanum to bring a high level down and then GFO to hold it, according to Bulk Reef Supply. Don’t start both at full strength at once.

Why is my phosphate zero but I still have algae?

Algae can use phosphate as fast as it appears, so a test shows zero. Remove the algae by hand and keep testing.